1. Field of the Invention
This invention relates generally to mass flow sensors and more particularly to a thermal dispersion mass flow meter employing resistance temperature detection elements coupled with a pressure transducer for refining the mass flow determined by the thermal sensors in accordance with relative pressure changes.
2. Discussion of the Related Art
Thermal dispersion mass flow meters are a common choice for flow metering devices in the commercial and industrial metering markets. A typical sensor element for use in such meters is the resistance temperature detector (RTD), the resistance of which is related to the temperature of the element itself. A typical sensor employs at least two RTD elements. One of them is referred to as a reference element and is normally unheated. The active RTD element is heated and the effect of mass flow on the heated element provides a measure of the flow velocity of the substance in the conduit being monitored. The density of the fluid, normally a gas, flowing across the active RTD is also a factor in the amount of heat dissipated from that RTD. In some measurement situations, fluid density will be a constant so it can easily be accounted for in the system.
Two different methods are commonly used in the thermal dispersion industry to determine the mass flow in a conduit. One is configured to maintain a constant temperature differential between the reference RTD and the active RTD. This method measures the voltage or current required to maintain the active RTD at a constant temperature above the reference RTD while heat is removed from the active RTD by way of the physical properties of the flowing fluid. The other method measures the voltage difference between the active and the reference RTD's while the active RTD is heated by a constant current or a constant power heat source. During this measurement, as with the other method, the active RTD loses heat by way of the physical properties of the flowing media.
There are many configurations of dispersion mass flow sensors, and more particularly, of heated RTD type sensors. An early such flow detector is shown in U.S. Pat. No. 3,366,942. This patent discloses a reference sensor, a heated or active sensor, and a separate heating element located closely adjacent the heated sensor element. The basic principal of operation of dispersion flow meters is discussed in this patent. A different configuration of a three-element thermal dispersion sensor is shown in U.S. Pat. No. 4,899,584. There are many other examples of detectors employing differential temperature sensors, some having three elements as described in the patents mentioned above, and some having two elements, where the active sensor has the heater integral therewith. Even a single element differential temperature sensor may be employed. The single element sensor works on a time sharing basis where it acts as a reference sensor part of the time and is then heated to act as the active sensor in relatively rapid succession.
Most of the known differential temperature sensors are configured with the reference and heated sensors arranged in parallel. They are mounted in the fluid conduit and project into the flow path as an insertion flow sensor. The sensor elements are positioned to permit unobstructed flow fluid past both the heated sensor and the reference sensor in such a way that one does not thermally influence the other. That means that the reference sensor must indeed be a reference with respect to the fluid being sensed without influence from the heat of the heated sensor or the fluid heated by the heated sensor.
It has been found that at low flow rates, approximately 1.5 feet per second (fps) or below, pressure changes within the conduit can make drastic differences in the mass flow rates determined by a thermal flow meter. For example, tests have shown that for an actual flow rate of about 1 fps, an increased pressure of 90 psi can cause an error in the thermal flow meter reading of more than 50%. Since there are practical instances where flow rates below 1.5 fps at elevated pressures must be determined with accuracy, conventional thermal mass flow meters have generally been insufficient for that purpose.